I think multiplayer would completely wreck the strategic / thoughtful gameplay of NetHack. When I'm playing, if I get into a sticky situation I'll frequently make perhaps one move per minute, because until I act the monsters can't act either, so I have lots of time to look through my potions and spells and decide on a plan. How does this translate in multiplayer?
It's not nethack, but I've made a multiplayer roguelike, that's actually launching its beta test today if you want to come play, best thing is to join the discord.
For multiplayer, you're better served turning to the "rogue-lite" genre of games. Spelunky in particular is great. Crypt of the Necrodancer, Dungeon of the Endless, and Enter the Gungeon are also very good.